In recent years, the number of users requiring a speedup of a radio communication system has been increasing and attention has been paid to a multicarrier transmission system represented by an OFDM system as one of systems capable of increasing the speed and capacity of it. The OFDM system arranges tens to thousands carriers at the shortest frequency interval theoretically causing no interference and transmits information signals in parallel by frequency division multiplexing. The OFDM system has an advantage of being hard to be influenced by multipath interference when the number of subcarriers to be used is increased, because of a symbol time longer than that of a single carrier system of the same transmission rate.
However, under a multipath environment, subcarriers undergo different amplitude changes and different phase changes, so that it is necessary to compensate theses changes when data is demodulated at the receiving side. As a method of compensating a propagation path, there is a method which modulates all or part of subcarriers by a code known between a transmitter and a receiver to transmit them as a pilot signal at the transmitting side, and estimates a propagation path change undergone by each of the subcarriers from a received pilot signal and compensates the estimated propagation path change at the receiving side. In this specification, a multicarrier signal including this pilot signal is referred to as “a symbol for propagation path estimation” hereinafter. Furthermore, the multicarrier signal is particularly referred to as “an OFDM symbol for propagation path estimation” when the signal formation is OFDM.
In estimation and compensation of a propagation path change, a time de-spread propagation path estimating method can be adopted which uses a Fourier transform and an inverse Fourier transform and takes advantage of the fact that a delayed profile signal is concentrated in some range of the output of the inverse Fourier transform to remove noise and interference (Patent Document 1). A gain obtained by the time de-spread propagation path estimating method is referred to as a time de-spread gain. In the OFDM system, the number of Fourier transform points (or the number of inverse Fourier transform points) used by a transmitter and a receiver seldom matches the number of subcarriers to which a signal to be transmitted is allocated (which is used for signal transmission) to correspond to making a guard band or filtering processing or the like in the transmitter. The number of Fourier transform points is usually 2n.
In particular, when the number of Fourier transform points is different from the number of subcarriers, a time de-spread gain is obtained, if a propagation path is estimated by a time de-spread propagation path estimating method, but a bad influence by a distortion has been remarkably seen on an end side of a band for which a frequency response is estimated according to a signal-to-noise power ratio. The distortion has a larger influence on an end side of a band for which a propagation path is estimated. For this reason, a technology which is an improved time de-spread propagation path estimating method has also been disclosed.
For example, in patent document 1, a method is disclosed which uses frequency information (frequency information from which noise, etc have been removed) for which a propagation path has been estimated by a time de-spread propagation path estimating method in the midsection of a band and uses frequency information from which noise, etc have not been removed for propagation path compensation with respect to a propagation path of a subcarrier located on an end side of the band. Furthermore, in patent document 2, a technology of easily distinguishing a reference element related to noise occurring on a transmission channel is disclosed. In other words, as a technology of distinguishing a reference element related to noise, setting a threshold when receiving, transmitting by increasing the power of the reference element, or a combination of them is disclosed. In other words, noise or interference can be removed by taking advantage of the fact that a delayed profile signal is concentrated in some range of an inverse Fourier transform output, and a distortion is prevented from occurring by also removing a signal power component when removing noise power from a delayed profile. According to this method, a precision frequency response can be calculated without being affected by a distortion or due to a time de-spread gain in the midsection of a band.
Furthermore, in these days, a multi-input/multi-output (MIMO) technology has been investigated actively. This technology transmits different data streams from different two or more antennas and distinguishes them to demodulate data by a receiver, and significantly contributes to an increase in transmission rate. A MIMO-OFDM system which is an OFDM system using a MIMO technology has also been investigated. In the MIMO-OFDM system, it is an important problem to estimate a propagation path between a transmitter and a receiver efficiently and accurately.
In non-patent document 1, a propagation path estimating method using carrier interferometry (CI) (referred to as “a CI method” hereinafter) is shown. One of features of this technology is that propagation paths from two or more transmitting antennas can be estimated with one OFDM symbol for propagation path estimation. The CI method is able to distinguish signals transmitted from the antennas by changing the amount of phase rotation of one signal for propagation path estimation and using different signals for propagation path estimation by the antennas. Thus, for example, when antenna 1 and antenna 2 are used and a propagation path from antenna 1 is calculated, a frequency response from antenna 1 can be calculated by removing a pulse corresponding to signal power from antenna 2 and performing frequency conversion. Likewise, when a propagation path from antenna 2 is calculated, a frequency response from antenna 2 can be calculated by removing a pulse corresponding to signal power from antenna 1 and performing frequency conversion.    Patent Document 1: Japanese Patent Laid-Open No. 2005-130485    Patent Document 2: Japanese Patent No. 3044899    Non-Patent Document 1: Kazunari Yokomakura et al., “Investigation about a propagation path estimation system using carrier interferometry in a MIMO-OFDM system”, The Institute of Electronics, Information and Communication Engineers, Technical Report RCS2005-79, August, 2005, p. 91-96.